1. Field of the Invention
The present invention relates to an ink jet recording apparatus for recording an image by discharging ink droplets to a recording medium.
2. Related Background Art
In a prior art so-called ink jet recording apparatus, a distance between an ink discharge plane of a recording head and a recording medium such as a recording sheet or a plastic sheet (sheet distance) greatly affects a quality of a recorded image. The shorter the sheet distance is, the more the image quality is improved, but if it is too short, the ink discharge plane may contact the recording sheet due to unevenness of the recording sheet, which deteriorates the image quality. A current target for the sheet distance is approximately 0.7 to 1.2 mm.
FIG. 4 shows a front view of a construction of a carriage 6 which mounts an ink jet cartridge 3 in such a recording apparatus. Numeral 1 denotes a recording head integrated with the ink jet cartridge 3, and numeral 2 denotes a carriage wheel attached to the carriage 6 to reciprocally move the carriage 6 while maintaining a constant distance between an ink discharge plane of the recording head 1 and a recording sheet 4 (sheet distance). The carriage wheel 2 is made of a material which prevents the transfer of ink even if it contacts a recording plane of the recording sheet after printing.
A positional relation between the recording head 1 and the carriage wheel 2 is such that the carriage wheel 2 is upstream of the recording head 1 as viewed in the direction of forward (direction A) printing. Thus, the recording sheet 4 is biased to the platen 5 by the carriage wheel 2 to keep a constant sheet distance when the carriage 6 is driven forwardly in the stationary state of the recording sheet 6 in which the recording sheet 6 is off the contact of a feed roller 12 and a pinch roller 13.
FIG. 10 shows a main part of the recording apparatus. The recording sheet 4 is fed while it is pinched by the feed roller 12 and the pinch roller 13. When the recording sheet 4 reaches a record position, a pulley 7 is rotated so that the carriage 6 is reciprocally driven perpendicularly to the direction of feed of the recording sheet along a guide rail 9 to conduct a recording scan. The recording scan is conducted in each of the forward movement and the backward movement of the carriage 6. When one recording scan is completed, the sheet is fed in accordance with a recording width of the recording head 1. This process is repeated several times to complete one page of image recording. A sheet eject roller 14 and a wheel 15 are arranged downstream of the recording apparatus to feed the recording sheet after the recording.
In the prior art apparatus, when a trailing end of the recording sheet 4 moves away from the contact of the feed roller 12 and the pinch roller 13, the recording sheet 4 is no longer biased by the platen 5 and the sheet distance becomes stable. Thus, in the bilateral printing, the recording sheet 4 is biased to the platen 5 in the forward printing because the carriage wheel 2 is upstream of the recording head 1 as viewed in the print direction so that the sheet distance may be maintained constant, but in the backward printing, the recording sheet 4 is not biased to the platen 4 because the carriage wheel 2 is downstream of the recording head 1 as viewed in the print direction so that the ink discharge plane may contact the recording sheet 4 and the image quality is deteriorated. Thus, the apparatus is controlled to stop the printing when the recording sheet 4 moves away from the contact of the feed roller 12 and the pinch roller 13. Accordingly, the print range is narrowed.